Complex effects of arachidonic acid and its lipoxygenase products on cytosolic calcium in GH3 cells.

The mechanisms by which arachidonic acid (AA) and its metabolites stimulate prolactin release from pituitary cells are not understood. Because Ca2+ ions are pivotal to exocytosis, we investigated the effects of AA metabolites on intracellular calcium concentration ([Ca2+]i) and membrane ionic currents using dual-excitation microspectrofluorimetry and whole cell patch-clamp techniques in GH3/B6 pituitary line cells. AA (1 microM) had a biphasic effect on [Ca2+]i, mobilization of intracellular Ca2+, followed by stimulation of Ca2+ entry. Only the latter appeared to result from the degradation of AA through the lipoxygenase pathway. Indomethacin (Indo, 10 microM) and lipoxygenase products 5-, 12-, and 15-hydroxyeicosatetraenoic acid (HETE, 1 microM), increased action potential duration (12-HETE) or frequency (5- and 15-HETE, Indo). These effects depended on inhibition of d-tubocurarine- or tetraethylammonium-sensitive K+ conductances and stimulation of voltage-dependent Ca2+ channels. Refilling of intracellular Ca2+ stores, and Ca2+ efflux, may also be stimulated. Our results demonstrate a control of [Ca2+]i by a second messenger (AA) and its metabolites (HETEs).